The present invention relates to a slot machine, and more particularly it pertains to a slot machine of the type having functions to control the stopping positions of plural series of symbols or indicia each sequentially being displayed and changed.
A slot machine is generally provided with a plurality of reels, on each of the outer periphery thereof a plurality of symbols being depicted with a constant interval between two adjacent symbols. By pulling a start lever after a playing coin is inserted, the reels are initiated into rotation simultaneously with each other. The reels are rotated for a certain time period and are stopped. When a combination of a set of symbols at a winning line coincides with the combination of a set of predetermined winning-prize symbols, coins (including tokens) are ejected, the number of which being determined in correspondence with the degree of difficulty in obtaining the combination. Recently, a television type slot machine has been sold in the market. The slot machine of this character uses a cathode ray tube in lieu of a mechanical reel. The television type slot machine displays a series of symbols which otherwise has been depicted on the mechanical reel, and has no mechanically moving part so that it can advantageously operate quietly. Slot machines are classified into two categories in accordance with the way reels are stopped. In one type, a stop button is provided for each reel for stopping the reel upon depressing the stop button, while in another type, a time duration for rotation of each reel is determined using a random timer, thereby in both cases causing the reels to stop one by one.
Generally, it is required to maintain a definite gain ratio for a game machine for business use, and also it is important to add a function to control the generation of winnings with the gain ratio being retained at a constant value. In a slot machine, coins are paid out when a winning-prize symbol combination is attained. Therefore, a pay-out ratio corresponds to the gain ratio described above. In conventional slot machines, a maximum value (maximum pay-out ratio) which permits the machine to pay out certain numbers of coins has been determined beforehand, and a pay-out ratio at a given time has been calculated from the number of coins inserted and ejected up to that time. Thus, the stopping positions of reels are controlled so as to make the pay-out ratio at a given time coincide with the maximum pay-out ratio. This reel stopping control is dependent upon the pay-out ratio at a given time, so that any one of the predetermined winning-prize symbol combinations cannot definitely be generated with a certain probability. For instance, a slot machine which pays an extraordinarily large number of coins, can seldom make a winning-prize combination, such as a set of symbols " 7" aligned in one winning line. Furthermore, in the case of a slot machine having a winning prize combination which results in an extremely large number of coins (dividends), a high dividend is difficult to obtain when a winning-prize combination with a low dividend has occurred with an appropriate ratio. As a result, a slot machine having a nature frequently generating a prize-winning combination with a low dividend, has a low probability to generate a winning-prize combination with a high dividend. Conversely, a slot machine having a nature generating a small number of winnings, has a high probability to generate prize-winning combination with a high dividend. Thus, a distorted nature of each slot machine may positively appear.